In days gone by people used to be content just using the shifter to feel and listen for the right place under a particular sprocket when changing gear. Now indexed gears are universal. The shifter has notches instead of moving smoothly across its range and, if all the components are compatible and correctly adjusted, shifting one notch on the shifter pulls through enough cable to move the chain across exactly one sprocket on your cassette.
Teeth that bite gently: shifting sprockets
A few derailleurs are designed to work in reverse – the cable pulls the chain from the largest to the smallest sprockets and, when the cable tension is released, the spring in the rear derailleur can pull the cable back from the smallest to the largest sprocket. The Shimano Rapid rise (or low normal) derailleur is like this. Some people prefer it, others find it irritating. The idea behind rapid-rise derailleurs is that the shift into a higher gear is controlled by cable action rather than spring action, so it’s supposed to be better for split-second changes in races. If your transmission is working perfectly, there is a slight advantage – but this is offset by the design’s inability to tolerate dirt or wear as well as the standard set-up.
Adjusting your gears
Well-adjusted gears should be invisible – one click of the shifter and you should move into whatever gear you need without thinking about it. You need to lavish care on your gears to keep everything running smoothly, though. Indeed, after keeping your chain clean, the next most important thing is to keep your gears well adjusted. They don’t simply work better – your entire transmission lasts longer. Get used to adjusting your gears before you tackle any other gear work, as you have to make adjustments at the end of many procedures, especially fitting new cables or derailleurs. The rear indexing is the most important adjustment – proper tuning is not difficult, but practice makes perfect.
There are a couple of things to bear in mind as you learn to adjust your gears. The first is that your gear adjustment depends on transferring an accurate signal from your shifters to your derailleurs, so that when you take up or release a length of cable at the shifter, exactly the same amount of cable is pulled through at the derailleur. This will not happen if the cable is dirty or frayed or the casing is kinked. If you find that the adjusting instructions aren’t working for you, check that cable and casing are in good condition. All these adjustments require being able to turn the pedals to change gear, so you need to hold your bike up so that the back wheel is off the ground. Ideally, use a workstand. Otherwise bribe a friend to lift it up by the saddle at appropriate moments.
In the picture on the right, you can see the chain in the middle of a gear shift from a larger sprocket to a smaller one. The chain, dragged across to the right by the derailleur, has to climb up and over the teeth of the larger sprockets before dropping onto the next sprocket down. Careful design of both the sprocket profile and the chain links means that the chain will shift under pressure, transferring from one sprocket to the next without loss of power. Shimano chains have side links that bulge outwards, allowing them to engage quickly with the sprocket teeth. Look carefully at
the faces of the sprockets on your cassette and you will see that these have also been cut away, making short ramps to help lift the chain up when shifting from smaller to larger sprockets. All these small design improvements, among others, help make your shifting instinctive and immediate.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Derailleurs”